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    Char-downcase-gen

    Downcase any character (even a non-standard one).

    ;; Convert CHAR from upper to lower case, if it is an upper-case letter.
;; Otherwise, return CHAR unchanged.
(defund char-downcase-gen (char)
  (declare (xargs :guard (characterp char)))
  (if (standard-char-p char)
      (char-downcase char)
    char))